This week I’m tackling the subject of our Sun’s motion through the Milky Way Galaxy and approximately how long one orbit is.
The Milky Way Galaxy has two major spiral arms, named the Perseus Arm and the Scutum-Centaurus Arm. There are also smaller less pronounced arms, including the Sagittarius Arm, the Norma Arm, The Local Arm (aka the Orion Spur) and the Outer Arm. Our solar system resides in the Orion Spur (Local Arm), branching off from the larger Perseus Arm. During the summer months in the northern hemisphere, we predominantly observe the Sagittarius Arm, including the galactic center, which appears as steam from the Tea Pot asterism in the constellation Sagittarius. (Gaherty, 2016) Over the winter, we’re looking away from the galactic center and through the Perseus Arm. (Comins, 396)
In this week’s discussion topic, I attempt to answer the question “Why are Uranus and Neptune distinctly bluer than Jupiter and Saturn?”
On Uranus and Neptune, the methane absorbs red, orange and yellow light, reflecting back the blue. In contrast, Jupiter and Saturn have only minor trace amounts of methane and quite a bit more hydrogen and ammonia.
Long answer: Still Jupiter, but let’s dive in and take a more detailed look.
Birth of a Gas Giant
A long time ago in a solar system very near you, just 1 or 2 AU past the snow line, enough surrounding planetesimals were accreted to become an Earth-like body containing about ten (10) Earth masses of metal and rock. This, in turn, gave this massive body enough gravitational attraction to pull vast amounts of hydrogen, helium and ices near its orbit, creating the first planet in our solar system: Jupiter. Impacts from the infalling gases and ices heated Jupiter up, so much so that for a short time, it outshown the protosun, if viewed from equal distances. Jupiter lacked the total mass to become a star, needing to be seventy-five (75) times more massive to achieve the necessary compression and heat in its core to sustain fusion.
My second post in my series of weekly discussion topics for my Introduction to Astronomy online class. Last week I got up close and personal with the many sides of the Moon. This week I take a closer look at the other blue planet in our solar system and how we discovered it without observing it first.
Why was the discovery of Neptune a major confirmation of Newton’s universal law of gravitation?
Before Newton, astronomy relied on observational data from which mathematical formulae and equations were created. Newton pioneered an approach which allowed mathematicians to extrapolate and predict the movement of objects using three assumptions, now commonly known as his laws of motion. Together with his formula for gravitational force, Newton transformed Kepler’s three laws to predict orbits of comets and other solar system objects. He further formulated a mathematical model, known as the Law of Universal Gravitation, that describes the behavior of the gravitational force that keeps the planets in their orbits. (Comins, 2015, p. 42-44)
Tomorrow, just after six o’clock in the morning and just as the sun is rising, we’ll experience the first full moon to occur on Christmas Day since 1977. I wasn’t even in high school yet in 1977 (although my husband was already in college by then). If you miss opening this Christmas present, you won’t get another chance until 2034 (by which time I should be retired).
Other astronomical items of note this holiday week include:
On the 4th day of Christmas (Monday that is), Mercury reaches its peak distance from the sun 30 minutes after sunset in the southwest.
On the 5th day of Christmas (Tuesday), Saturn continues its return from behind the Sun. Look to the southeast in the pre-dawn morning time.
On the 6th day of Christmas (Wednesday), look up and south to spy the Seven Sisters (aka as the Pleiades)
On the 8th day of Christmas (Happy New Year!), use binoculars to find Comet Catalina rising close to Arcturus (a very bright star) around midnight and continue to rise high in the southeast until dawn twilight.
On the 9th day of Christmas (Saturday, January 2, 2016) the Earth reaches its closest point to the Sun (at the start of Winter no less)
On the last day of Christmas (Twelfth Night) at 10 p.m. EST, Pluto hides behind the Sun.